Anomalous diffusion in presence of twin boundaries

After implantation of the ⁷¹Ge radiotracer in the Mg-1.8 wt% Al magnesium alloy, tracer diffusion coefficients were measured using both the serial sectioning and the residual activity technique in the temperature range from 448 to 848 K. Anomalously high diffusivities were observed at high temperatures where volume diffusion usually predominates. A contribution of high diffusivity paths to the total diffusion flux can be a possible explanation of both phenomena. Based on the high fraction of twin boundaries confirmed by electron microscopy, a simple microstructure model is proposed.     

Effect of dislocation distribution in twin boundaries on the nucleation of microcracks at the twin tip

The effect of dislocation distribution in the boundaries of an arrested twin on the nucleation of microcracks at its tip is investigated. The twin is simulated by a double step pileup (cluster) of twinning dislocations located in adjacent slip planes. The equilibrium equations for dislocations are solved numerically. Clusters with different total numbers of dislocations and with different ratios of the numbers of dislocations at the upper and lower twin boundaries are considered. The formation of microcracks as a result of coalescence of head dislocations according to the force and thermally activated mechanisms is analyzed. The equilibrium configurations of a single twin boundary and of the twin are calculated. It is found that the condition for microcrack formation at the twin tip considerably depends on the ratio of the numbers of dislocations in twin boundaries. In the limit, this condition coincides with the condition of crack formation at the tip of a single twin boundary with the same total number of dislocations. It is shown that thermally activated formation of a microrack corresponds to lower values of the critical stress.     

Cleavage cracking across twin boundaries in free-standing silicon thin films

Cleavage cracking across twin boundaries in free-standing silicon thin films is investigated in a microtensile fracture experiment. If the twist misorientation is relatively small, the crack front transmission can be quite smooth; otherwise the fracture surface may be either planar or broken down into parallel terrains. In all the cases, the local fracture resistance tends to increase. PACS 62.20.-x; 68.35.bg; 68.35.Gy; 68.37.-d; 62.20.mm     

Direct characterization of boron segregation at random and twin grain boundaries

Boron distribution at grain boundaries in hot-deformed nickel is directly characterized by the time-of-flight secondary ion mass spectrometry. The segregations of boron are observed at both the random and twin grain boundaries. Two types of segregations at random grain boundaries are observed. The first type of segregation has a high intensity and small width. Its formation is attributed to the incorporating of dislocations into the moving grain boundaries. The second type of segregation arises from the cooling induced segregation at the dislocations associated with the grain boundaries. The segregation at twin boundary is similar to the second type of segregation at random grain boundaries.     

Pinning by twin boundaries and peak effect in YBaCuO high-T c superconductors

Measurements of the imaginary part of the ac magnetic susceptibility of single crystals and melt-textured samples of YBa₂Cu₃O_x (YBCO) at T=77 K in a magnetic field ranging between 1 and 20 kOe are reported. If the dc magnetic field H _dc is rotated in the ab plane of the sample, the magnetic susceptibility and critical current density j _c have peaks corresponding to the magnetic field aligned with twin boundaries. Peaks in the curve of j _c versus magnetic field are observed at angles corresponding to these peaks, where AH _dc in a wide range of magnetic fields. The results have been interpreted in terms of the theory describing twin boundaries as a system of quasi-planar pinning sites. The pinning is strong if the elastic displacements of flux lines are of the order of the vortex lattice constant d _f. These displacements decrease with the magnetic field because of the decrease in d _f, and the contribution of the elastic energy to the Gibbs potential is reduced accordingly, which is the cause of the peak effect. Zh. éksp. Teor. Fiz. 111, 2158–2174 (June 1997)     

Vortex lattice accommodation on twin boundaries in studied by neutron diffraction

An extensive study of small angle neutron scattering was performed in twinned YBa₂Cu₃O₇ crystals in its superconducting state as a function of the angle between the c-axis of the crystal and the magnetic field. The half of the twin boundaries are oriented in the horizontal plane, which also contains the neutron beam and the magnetic field. Two different diffraction patterns are studied as a function of at 5 K and B = 0.5 T, one along the c-axis of the crystal, the other one along the applied field. These variations are interpreted in the model of accommodation of the vortices on the twin planes by zigzagging from these planes to the ab-planes of the crystal, in order to minimize their energy. Received: 9 March 1998 / Revised and Accepted: 12 June 1998     

Electronic properties of PbS(111) twin boundaries and twinning superlattices

We have studied the electronic properties of single twin boundaries and twinning superlattices in lead sulfide. The results are compared against those obtained previously for such structures based on diamond/zincblende type materials, and substantial differences are noted. It is found that lead sulfide twinning superlattices can have significantly wider or narrower band gaps than the homogeneous material, depending on whether the twin interface is occupied by anion or cation atoms.     

Production of materials saturated with twin boundaries by compacting electrolytic microparticles

A technique is proposed for producing nanostructured materials saturated with twin boundaries by compacting (pressing) small pentagonal electrolytic particles. The concentration of twin boundaries in the formed compound is estimated.     

Magnetically modulated microwave absorption in twin boundaries of a high-temperature superconducting single crystal

We propose a model of magnetically modulated microwave absorption (MMMA) in twin boundaries of high-temperature superconductors which assumes formation of superconducting current loops closed by two Josephson junctions. It well explains the origin and properties of the periodic signal in HTS monocrystal.     

Magnetism in Fe/V superlattices and alloys

The first principles discrete variational X_α method was used to calculate the electronic structures, magnetic moments and the conduction electron contact spin densites at a central V site in a number of 15 atom clusters representing [110]-interfaces in Fe/V superlattices and alloys. It was found that interactions between the spin-up and spin-down 3d electrons behave differently as the number of Fe atoms, P, in the first coordination shell of a central V atom changes. The 3d magnetic moments at the V sites were found to vary strongly with P, and their variation with respect to changes in the number of Fe atoms in the second shell, Q, was found to depend on P. The sign of 4s and 4p moments was found to change from positive to negative as the average 3d magnetic moment per atom at the first shell sites goes from negative to positive. The conduction electron contact spin densities follow the pattern of the 4s moment as P varies. The results of previously calculated Fe-centered and the present V-centered clusters are combined to study the average magnetic moment in iron-vanadium superlattices and alloys.     

Spectra and multiplicities of n, p, d, t, K, π from antiproton annihilation in Cu and U

Inclusive energy spectra of neutrons, protons, deuterons, tritons, charged kaons and pions following antiproton annihilation at rest in copper and uranium have been measured. The determined multiplicity charged kaons is compared to recent measurements and indicates an enhancement of strangeness production. Ratios of directly emitted neutrons and protons have been determined for copper and uranium to 3.0±0.6 and 3.2±0.5, respectively.